Introduction

An effective training program for distance runners requires repeated cycles of physiological stress and recovery, leading to an adaptation to speed and mileage loads. Specific workouts can be designed to improve the following three key physiological parameters: VO2max, anaerobic threshold (AT) and aerobic endurance. There are many other factors such as efficiency, psychology, heat-tolerance etc. which also contribute to running performance but the above three are essential to success.

VO2max

This represents the maximum rate of oxygen consumption for an individual and is measured in mls/minute/kg body-weight. Oxygen is the terminal electron acceptor in the energy-generating process of aerobic respiration which occurs in the mitochondria of exercising muscles. Therefore VO2max determines the upper limit of energy available to muscles. This upper limit is largely genetically determined but training can increase it by up to 20% of non-trained capacity.
For most trained runners, running at 5K race pace approximates to VO2max effort, therefore intervals of 400m to 1600m close to 5K race pace are highly effective in improving this parameter. The pace calculator above computes 400m, 800m, 1200m and 1600m interval times corresponding to 102%, 100%, 98% and 96% ,respectively, of VO2-Max effort.
There are four variables to be considered when doing interval training in general:

• Length of the interval.
  This depends on the race distance that you are training for. As a general rule, longer intervals should be emphasized when training for longer events.

  Race Distance	Interval Distance
  5K	200 -800m
  10K	400-1200m
  Half-Marathon	400-2000m
  Marathon	800-3200m

  
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• Speed of the interval
  Interval times should be based on current 5K pace as described above with pace decreasing by about 2% per 400m increase in distance (obviously, longer intervals are more stressful!). The pace of these intervals should be adjusted for factors such as heat, humidity, altitude, headwinds, pollen count, etc. which make fast running subjectively more difficult. Don't get locked in to the idea that you must run VO2-Max intervals at a particular pace, irrespective of environmental conditions. This is where a heart-rate monitor comes in handy because it provides a more objective assessment of effort.
  
  

• Number of intervals
  A good rule of thumb is to run no more than 10% of your weekly mileage at a fast pace. For example, a runner averaging 25 miles (40K) per week could do a VO2-Max workout consisting of 5 x 800m or 10 x 400m (total of 4K) once a week. Of course, racing also qualifies as a VO2-Max workout so take that into account too!
  
  

• Length of the recovery between intervals
  The recovery is what makes interval running easier than racing (assuming that you do both at the same pace) because it allows lactic acid to be flushed from your working muscles and a more aerobic state to be maintained. For distances up to 800m, a recovery period equal to the time taken for the preceding interval is appropriate. For example, if you are doing 400m repeats in 80 seconds, do a 80-second jog or walk in between. For longer distances, a recovery of 3-4 minutes is usually sufficient to allow the heart-rate to drop to about 50% of maximum. As an alternative to using a fixed recovery time, a heart-rate monitor may conveniently be used to define the length of the recovery by waiting until the rate drops to ,say, 120 bpm before commencing the next repeat. With increased fitness, the length of the recovery period can be decreased to stress the aerobic system further and thereby approximate racing conditions more closely (where you don't have the luxury of any rest periods!).

In summary, this type of interval training increases maximal oxygen uptake by increasing cardiac stroke-volume, increasing the size and number of energy-producing mitochondria in working muscles and improving the oxidative capacity of fast-twitch muscle fibers. It also tends to improve biomechanical efficiency and lactate tolerence as well as promoting mental toughness.
One last word of advice: if you're sick or injured, avoid this type of workout- it's likely to do more harm than good.

Anaerobic Threshold (AT)

During easy running the supply of oxygen is sufficient to ensure that aerobic ("with oxygen") metabolism is the predominant energy-producing pathway for the working muscles. You're breathing is easy, your legs feel good and everything's cool! As running pace is increased, the amount of available oxygen is no longer sufficient to meet the body's energy demands and a second pathway called anaerobic ("without oxygen") glycolysis is recruited. The end product of anaerobic glycolysis is lactic acid (lactate). As running pace is increased futher, the lactate concentration in the exercising muscles increases rapidly and this point is referred to as tha anaerobic threshold (AT) or lactate turnpoint. Subjectively, this is where a pace at which breathing becomes more labored and the dreaded burning sensation in the legs begins to appear. Well-trained athletes usually reach their AT at approximately 85-90% of their VO2max heart-rate but for untrained individuals this threshold is much lower (50-70% of VO2max heart-rate). In terms of running speed, AT pace for trained runners approximates to half-marathon pace and is typically 20-40 seconds/mile slower than VO2-Max pace. Workouts at AT pace (also known as tempo runs) usually involve repeats of 800m to 3200m or alternatively a single run of 2 to10 miles, depending on conditioning and experience.
Running at AT pace increases anaerobic threshold (i.e. a runner can attain a higher percentage of VO2-Max heart-rate before going anaerobic). It also simulates race-pace speed for longer races such as the half-marathon and marathon, without stressing the body to the extent that VO2-Max training does.

Aerobic Endurance

Easyrunning at 60 -75% of maximum heart rate comprises the largest portion (75-85%) of the runners weekly training mileage. In the case of beginners, this should be the only form of training for the first 4-6 months. Running at this pace is almost entirely aerobic with fats being the predominant energy source and glycogen being utilzed to a lesser extent. Harder VO2-Max and AT workouts are interspersed with Aerobic Endurance training to follow a "hard day/easy day" routine, allowing the body to recover and adapt to the stress of faster-paced workouts. One particularly important form of aerobic endurance training is the weekly long run which improves the fat-burning (and therefore glycogen-sparing) ability of the body and increases the tolerance of the muscles, tendons, skeletal system to prolonged exercise. The optimal distance for the weekly long run varies considerably with the racing distance that one is training for and the table below provides some broad guidelines:

Preparing a Schedule